In recent years, from the standpoint of global environmental protection, efforts have been made to reduce the weight of automotive bodies and improve the mileage of automobiles. The improvement in the mileage of automobiles has also been required by law. Recently, efforts have been made to use high-strength materials as the materials for automotive body to reduce the weight of automobiles by gauge down (thickness reduction). Furthermore, improvement in the stiffness of members using a closed-cross-section structure is under study. In response to the improvement in the stiffness of automotive members, high-strength steel tubes began to be used.
As in steel sheets, such high-strength steel tubes are essentially required to be easy to process and have excellent chemical conversion treatability. In general, high-strength steel tubes are basically designed to contain 0.7% by mass or more Si to achieve both high strength and excellent formability. However, the inclusion of Si is inevitably accompanied with a marked deterioration in chemical conversion treatability. The mechanism of deterioration in the chemical conversion treatability of steel materials having a high Si content is known to some extent as described below.
In steel materials containing Si, an oxide mainly composed of Si is concentrated on a surface layer of the steel material (other equivalent expressions, such as a Si-based oxide, a Si-containing oxide, a Si oxide, and a Si group oxide, mean the same oxide; unless otherwise specified, these are collectively referred to as an oxide mainly composed of Si). An oxide mainly composed of Si prevents Fe in a base steel material from uniformly dissolving as Fe2+ and inhibits the formation of iron-zinc phosphate crystals (Zn2Fe(PO4)2.4H2O) in the anode reaction and cathode reaction during chemical conversion treatment. Thus, dense and fine iron-zinc phosphate crystals cannot be formed on the steel material. As illustrated in FIG. 1, the chemical conversion treatment of high-Si steel results in the formation of iron-zinc phosphate crystals having coarse and sparse iron-zinc phosphate crystal-free areas (hereinafter referred to as crystal-free areas). In contrast, as illustrated in FIG. 2, the chemical conversion treatment of mild steel having a low Si content (JIS-SPCC-grade steel sheets) forms very dense iron-zinc phosphate crystals.
In cold-rolled steel sheets, pickling of a hot-rolled steel sheet before cold rolling can partly remove an oxide mainly composed of Si. However, in cold-rolled steel sheets subjected to an annealing process, such as continuous annealing or batch annealing, after cold rolling, an oxide mainly composed of Si is again inevitably concentrated on a surface layer in a furnace even at a very low dew point. Thus, cold-rolled steel sheets also often have poor chemical conversion treatability. Furthermore, in the annealing process, gradual variations in the environment within the furnace, variations in the components of steel, or variations in manufacturing conditions often result in variations in the distribution of an oxide mainly composed of Si from one coil to another in the longitudinal and width directions of the coil. In the formation of an oxide mainly composed of Si, variations in the components of steel, variations in manufacturing conditions, and the like intricately interact with one another. It is therefore difficult to manage these influencing factors to control chemical conversion treatability.
Thus, the surfaces of steel sheets manufactured have hitherto been ground in a mechanical process or dissolved in a chemical process, such as pickling, to remove an oxide mainly composed of Si that inhibits chemical conversion. For example, PTL 1 describes a method for manufacturing high-tensile steel sheets with a high Si content having excellent phosphate coating treatability. This method includes annealing in an atmosphere in which the oxygen partial pressure is controlled within a particular range, quenching in a particular temperature range, grinding of the surface, and pickling to remove an oxide film.
PTL 3 describes a method for manufacturing high-strength cold-rolled steel sheets having excellent chemical conversion treatability. This method includes softening and annealing of cold-rolled steel sheets having a (Si content)/(Mn content) of 0.4 or more in an atmosphere at a dew point in the range of −20° C. to 0° C. such that the fraction of surface coverage of a Si group oxide is 20% or less and the equivalent circular diameter of the Si group oxide is 5 μm or less, water quenching, tempering, and immersion in hydrochloric acid or sulfuric acid for pickling.
PTL 12 describes a method for manufacturing high-strength electric-resistance-welded steel tubes having excellent chemical conversion treatability. This method includes hot-rolling and pickling of a steel sheet having a composition of Si: 0.5% by mass or less, Mn: 1.5% by mass or less, and P: 0.05% by mass or less to remove an outer surface layer and an inner surface layer, cold rolling at a cold-rolling reduction in the range of 10% to 60%, and electric-resistance welding (ERW) of both ends of the cold-rolled steel strip in the width direction to form a welded steel tube.
However, grinding or pickling requires a large number of man-hours, and it is difficult to completely remove an oxide mainly composed of Si. Furthermore, an oxide mainly composed of Si is glass and consequently does not dissolve in a common acid, such as hydrochloric acid or sulfuric acid. Furthermore, since an oxide mainly composed of Si cannot be selectively removed by pickling, a base steel sheet must be significantly dissolved to remove the oxide mainly composed of Si.
PTL 2 describes a method for treating a steel surface, which includes immersion of a steel material in a mixed acid of sulfuric acid and hydrofluoric acid at a sulfate ion concentration and a hydrogen fluoride concentration in particular ranges and subsequent immersion of the steel material in hydrochloric acid at a chloride ion concentration in a particular range. Although pickling in a fluorinated acid type agent can completely remove an oxide mainly composed of Si, use of the fluorinated acid type agent may somewhat increase the degree of danger.
PTLs 4 to 8 describe a technique for improving chemical conversion treatability by forming a Si—Mn composite oxide easily soluble in an acid while preventing the formation of a slightly soluble oxide mainly composed of Si.
PTL 4 describes a multiphase steel sheet having excellent coating adhesion in which the Si and Mn contents are controlled so as to satisfy a Si/Mn ratio of 0.4 or less, there are 10 or more fine Mn—Si composite oxide particles containing 0.5% by mass or more (Mn—Si) on a surface layer (an area 2 μm in depth and 10 μm in length), and an oxide mainly composed of Si accounts for 10% or less of the surface length of the steel sheet.
PTL 5 describes a multiphase high-strength cold-rolled steel sheet having excellent coating adhesion in which the Si and Mn contents are controlled so as to satisfy a Si/Mn ratio of 0.4 or less, there are 10/100 μm2 or more fine Mn—Si composite oxide having a Mn/Si ratio of 0.5 or more, the fraction of surface coverage of an oxide mainly composed of Si is 10% or less, and there is no crack having a size in a predetermined range.
PTL 6 describes a multiphase high-strength cold-rolled steel sheet having excellent strength-elongation balance, that is, a high elongation/strength ratio, wherein the Si and Mn contents are controlled so as to satisfy a Si/Mn ratio of 0.4 or less, there are 10/100 pre or more fine Mn—Si composite oxide having a Mn/Si ratio of 0.5 or more, the fraction of surface coverage of an oxide mainly composed of Si is 10% or less, and the tensile strength is 390 MPa or more.
PTL 7 describes a high-strength steel sheet having excellent coating adhesion in which the average distance between the starting points of Si- and/or Mn-containing oxide stemming from a surface of the steel sheet in the depth direction in a network-like or hair-root-like manner is 5 μm or more, and the total length of the oxide is 10 μm/(12 μm in depth×20 μm in width) or less.
PTL 8 describes a Si—Mn oxide multiphase high-strength steel sheet having excellent coating adhesion in which the Si and Mn contents are controlled so as to satisfy a Si/Mn ratio of 0.4 or less, there are 10/100 μm2 or more fine Si—Mn oxide on the surface, and the fraction of surface coverage of an oxide mainly composed of Si is 10% or less.
Although a Si—Mn composite oxide adversely affects chemical conversion treatability as with an oxide mainly composed of Si, the Si—Mn composite oxide easily dissolves in an acid. In the techniques described in PTLs 4 to 8, therefore, a Si—Mn composite oxide is intended to be removed by “in-line pickling”, which is often provided in the production lines of cold-rolled steel sheets.
However, in the techniques described in PTLs 4 to 8, since the Mn content depends on the Si content, there is a problem of a limited degree of freedom in the design of steel components. There is also a problem that improvement in chemical conversion treatability is often limited.
It is known that zinc phosphate treatment for use in mechanical lubrication, which can be used in combination with a lubricant to facilitate plastic working, can be subjected to shot blasting as pretreatment to improve chemical conversion treatability. For example, PTL 9 describes a method for forming a conversion coating on a surface. The method includes ejecting a zinc phosphate chemical conversion treatment liquid to which silica sand has been added against the surface to clean the surface and then ejecting the zinc phosphate chemical conversion treatment liquid. It is assumed that the mechanism by which shot blasting before chemical conversion treatment can improve chemical conversion treatability is due to the mechanochemical activation of a surface by shot blasting (see NPL 1). However, leaving a shot-blasted surface to stand in the air or annealing a shot-blasted surface reduces the mechanochemical activity of the surface, failing to achieve a desired improvement in chemical conversion treatability.
Even when shot blasting is employed as pretreatment of coating, in consideration of actual physical distribution, a considerable amount of time elapses from the shot blasting to coating in the manufacture of steel sheets and steel tubes. In practical terms, therefore, the effects of improving chemical conversion treatability are markedly reduced and are not thought to be significant. The employment of continuous in-line shot blasting to reduce the time elapsed from shot blasting to coating requires considerable costs and therefore has a low degree of realizability.
PTL 10 describes a high-tensile hot-rolled steel sheet having excellent chemical conversion treatability and corrosion resistance, wherein the steel sheet contains 0.5% to 2.5% by mass Si and contains C and Ti such that C and Ti satisfy a particular relationship, the average grain diameter is 3.0 μm or less, and the surface roughness is controlled to 1.5 μm or less as an arithmetical mean roughness Ra. In accordance with the technique described in PTL 10, the small crystal grain diameter and the smooth surface result in a marked improvement in chemical conversion treatability.
NPL 2 has reported that the surface roughness of a steel sheet does not significantly affect chemical conversion treatability at Ra in the range of 0.5 to 1.7 μm, PPI in the range of 110 to 250, or Wz in the range of 1 to 8 μm.
PTL 11 describes a method for manufacturing cold-rolled steel sheets that can effectively improve phosphate treatability without impairing the press formability of the steel sheets. The method includes annealing of a steel sheet containing 0.01% by mass or less C, 0.01% by mass or less N, and Ti and skin pass rolling at a rolling reduction of 0.8% or more and 5% or less. In accordance with PTL 11, the chemical conversion treatability is saturated at a rolling reduction of 2.7% or more in the skin pass rolling.